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### Heat Gain or Loss HVAC Duct ASHRAE Formula and Calculator

Heat Gain or Loss HVAC Duct ASHRAE Formula and Calculator

Alternative resource for calculating heat loss or gain: Heat Loss from Ducts Equations and Calculator

All HVAC ducts exposed to outdoor conditions, as well as those passing through unconditioned or semiconditioned spaces, should be insulated. Analyses of temperature change, heat loss or gain, and other factors affecting the economics of thermal insulation are essential for large commercial and industrial projects. ASHRAE Standard 90.1 and building codes set minimum standards for thermal efficiency, but economic thickness is often greater than the minimum. Additionally, the standards and codes do not address surface condensation issues. These considerations are often the primary driver of minimum thickness in unconditioned or semiconditioned locations subject to moderate or greater relative humidity.

Duct thermal efficiencies are generally regulated by local or national codes by specifying minimum thermal resistances, or R-values. These R-values are most often determined by testing per ASTM Standards C518 or C177, as required by the Federal Trade Commission (FTC) for reporting R-values of duct wrap insulations. Neither method allows for increased thermal resistance caused by convective or radiative surface effects. To comply with current code language, it is recommended that R-value requirements in specifications for duct insulation be based on Standards C177 or C518 testing at 75°F mean temperature and at the installed thickness of the insulation. Insulation products for ducts are available in a range of R-values, dependent predominantly on insulation thickness, but also somewhat on insulation density.

Heat loss programs based on ASTM Standard C680 may be used to calculate thermal energy transfer through the duct walls. Duct air exit temperatures can then be estimated using the following equations:

Heat Gain or Loss HVAC Duct ASHRAE Formula:

tdrop or tgain = 0.2 [ ( q · P · L ) / ( V · Cp · ρ · A ) ]

for warm air ducts,
texit = tenter - tdrop

for cold air ducts,
texit = tenter - tgain

Where:

tdrop = temperature loss for warm air ducts, °F
tenter = entering air temperature, °F
tgain = temperature rise for cool air ducts, °F
texit = exit temperature for either warm or cool air ducts, °F
q = heat loss through duct wall, Btu/h·ft2
P = duct perimeter, in.
L = length of duct run, ft
V = air velocity in duct, ft/min
Cp = specific heat of air, Btu/lbm·°F
ρ = density of air, 0.075 lb/ft3
A = area of duct, in2
0.2 = conversion factor for length, time units

Related:

Reference:

• ASHRAE Fundamental Handbook, 2019